How do I find a child’s blood type from the parents?

Choose both parents’ blood type and Rh — by the laws of inheritance the calculator shows the child’s possible types and their approximate probability. The exact type is determined by a blood test after birth.

Can type O parents have a child of another type?

No. Two parents with type O (I) always have an O (I) child: they have no A or B allele to pass on. A different type would be a reason for genetic counselling.

Can Rh+ parents have an Rh− child?

Yes. An Rh-positive person can silently carry a negative gene. If both parents pass it on, the child is Rh−. So an Rh− child of two Rh+ parents is normal.

It’s when the mother is Rh− and the fetus is Rh+ (from an Rh+ father). The mother’s body may make antibodies to the child’s Rh factor. Such pregnancies are monitored closely; anti-D immunoglobulin is given to prevent it. The calculator flags the possibility.

It’s a probability estimate by the laws of inheritance, not a guarantee. A visible type doesn’t always reveal the parents’ hidden alleles, plus there are rare exceptions. Only a lab test gives the exact type and Rh.

Child’s blood type from parents

Choose the mother’s and father’s blood type and Rh — the calculator shows which blood types and Rh a child can inherit and with roughly what probability, and warns about a possible Rh conflict. Based on the laws of inheritance (ABO and Rh).

Work out the child’s blood type and Rh

Choose the blood type and Rh of both parents — the result appears instantly.

Blood types and notation

Blood type is defined by A and B antigens, Rh by the D antigen. Russian and international notation match.

Type	Notation
O (I)	no A or B antigens
A (II)	antigen A
B (III)	antigen B
AB (IV)	antigens A and B

How blood type is inherited

Blood type is defined by two gene alleles — one from each parent. A and B alleles are dominant, O is recessive. So type A can be “pure” (AA) or silently carry O (AO); the same for B. A child gets one allele from each parent, and the combination sets its type.

For example, parents with types A and B can have a child of any of the four types, depending on which hidden alleles they pass on. But two parents with type O will always have an O child.

Rh factor and Rh conflict

The Rh factor is the D antigen on red cells: present (Rh+) or absent (Rh−). Rh+ is dominant, so two Rh+ parents can still have an Rh− child if both pass the “hidden” negative gene.

An Rh conflict is possible when the mother is Rh− and the child is Rh+ (inherited from an Rh+ father): the mother’s immune system may react to the fetus’s Rh-positive blood. Such a pregnancy is managed specially, and anti-D immunoglobulin is given if needed. The calculator flags this situation.

Why it’s an estimate, not a guarantee

A visible type doesn’t always reveal whether a gene is “pure” or hidden, so the probabilities are approximate. There are also rare exceptions — such as the Bombay phenotype or cis-AB — where inheritance looks unusual. Only a lab test determines the exact blood type.

Frequently asked questions

Choose both parents’ blood type and Rh — by the laws of inheritance the calculator shows the child’s possible types and their approximate probability. The exact type is determined by a blood test after birth.
No. Two parents with type O (I) always have an O (I) child: they have no A or B allele to pass on. A different type would be a reason for genetic counselling.
Yes. An Rh-positive person can silently carry a negative gene. If both parents pass it on, the child is Rh−. So an Rh− child of two Rh+ parents is normal.
It’s when the mother is Rh− and the fetus is Rh+ (from an Rh+ father). The mother’s body may make antibodies to the child’s Rh factor. Such pregnancies are monitored closely; anti-D immunoglobulin is given to prevent it. The calculator flags the possibility.
It’s a probability estimate by the laws of inheritance, not a guarantee. A visible type doesn’t always reveal the parents’ hidden alleles, plus there are rare exceptions. Only a lab test gives the exact type and Rh.

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This calculator is for reference and information only and gives a probability estimate by the laws of inheritance. The exact blood type and Rh are determined by a lab test; for questions about Rh conflict, see a doctor.